As more functional blocks, with varying activity rates, are integrated into a microprocessor, large temperature variations appear across the microprocessor's substrate. This temperature variation can adversely affect the microprocessor's performance and reliability. To counter this non-ideality, people have introduced a scheme where they first measure the spatial thermal profile of a microprocessor and subsequently adjust a local power dissipation adapted to the measured local temperature. The spatial thermal profiling of a microprocessor may be executed by building integrated temperature sensors, often termed smart temperature sensors, into various locations within the chip.
There exist two types of smart temperature sensors. The first type, which represents a well-established, and very successful paradigm for smart temperature sensors, utilizes the temperature dependence of bandgap voltages of bipolar transistors. For example, the proportional-to-absolute-temperature (PTAT) bandgap voltage of bipolar transistors is measured using an analog-to-digital converter (ADC) to generate a digital temperature output as shown in FIG. 1A. In standard CMOS processes, parasitic pnp junctions involving the substrate are utilized to form bipolar transistors.
The second type of smart temperature sensors, which have appeared more recently and have significant possibilities for further development, are based on CMOS transistors. Exploiting the fact that the time delay of a CMOS inverter varies with temperature, the CMOS smart temperature sensors measure delays in open-loop inverter chains as shown in FIG. 1B, or similarly, frequencies of ring oscillators, to extract temperature information. The time delay can be measured using a time-to-digital converter (TDC) to produce a digital temperature output. For microprocessor spatial thermal profiling, these delay-based CMOS smart temperature sensors may potentially be well suited, as, though less accurate, they are all digital and use only standard CMOS transistors without the need for parasitic bipolar transistors.